How does this medication work? What will it do for me?

Methimazole belongs to the class of medications called antithyroid medications. It is used to treat hyperthyroidism (overactive thyroid gland). It prevents the thyroid gland from over-producing thyroid hormone, but does not interfere with the actions of thyroid hormone. It may take weeks to months before methimazole has its full effect on the symptoms of overactive thyroid (e.g., palpitations, sweating). During this time period, other medications may be used to control these symptoms.

Your doctor may have suggested this medication for conditions other than those listed in these drug information articles. As well, some forms of this medication may not be used for all of the conditions discussed here. If you have not discussed this with your doctor or are not sure why you are taking this medication, speak to your doctor. Do not stop taking this medication without consulting your doctor.

Do not give this medication to anyone else, even if they have the same symptoms as you do. It can be harmful for people to take this medication if their doctor has not prescribed it.

What form(s) does this medication come in?

5 mg
Each round, white, scored tablet, marked with "J94" on one side and plain on the other, contains methimazole 5 mg. Nonmedicinal ingredients: corn starch, lactose monohydrate, magnesium stearate, and talc.

10 mg
Each round, white tablet, marked with "10" on one side and plain on the other side, contains 10 mg methimazole. Nonmedicinal ingredients: corn starch, lactose monohydrate, magnesium stearate, and talc.

How should I use this medication?

Adults: Methimazole is taken 3 times daily, every 8 hours. The recommended starting total dose of methimazole is 15 mg daily for mild hyperthyroidism, 30 mg to 40 mg daily for moderately severe hyperthyroidism, and 60 mg daily for severe hyperthyroidism. The eventual maintenance dose is normally 5 mg to 15 mg daily.

Children: For children, the daily dose is initially 0.4 mg per kilogram of body weight, divided into 3 doses and given every 8 hours. The eventual maintenance dose is usually one-half the initial dose.

Methimazole can be taken with or without food.

Many things can affect the dose of medication that a person needs, such as body weight, other medical conditions, and other medications. If your doctor has recommended a dose different from the ones listed here, do not change the way that you are taking the medication without consulting your doctor or pharmacist.

It is important to take this medication exactly as prescribed by your doctor. The 3 daily doses should be taken as close to every 8 hours as possible so that blood levels are kept as constant as possible. If you miss a dose of this medication, take it as soon as possible and check with your doctor or pharmacist.

Store this medication at room temperature, protect it from light and moisture, and keep it out of the reach of children.

Do not dispose of medications in wastewater (e.g. down the sink or in the toilet) or in household garbage. Ask your pharmacist how to dispose of medications that are no longer needed or have expired.

Who should NOT take this medication?

Methimazole should not be taken by anyone who:

  • is allergic to methimazole or to any of the ingredients of the medication
  • is breast-feeding

What side effects are possible with this medication?

Many medications can cause side effects. A side effect is an unwanted response to a medication when it is taken in normal doses. Side effects can be mild or severe, temporary or permanent. The side effects listed below are not experienced by everyone who takes this medication. If you are concerned about side effects, discuss the risks and benefits of this medication with your doctor.

The following side effects have been reported by at least 1% of people taking this medication. Many of these side effects can be managed, and some may go away on their own over time.

Contact your doctor if you experience these side effects and they are severe or bothersome. Your pharmacist may be able to advise you on managing side effects.

  • dizziness
  • loss of taste sensation
  • nausea
  • stomach pain
  • vomiting

Although most of these side effects listed below don't happen very often, they could lead to serious problems if you do not check with your doctor or seek medical attention.

Check with your doctor as soon as possible if any of the following side effects occur:

  • backache
  • black, tarry stools
  • blood in urine or stools
  • increase in bleeding or bruising
  • increase or decrease in urination
  • numbness or tingling of fingers, toes, or face
  • pinpoint-sized red spots on skin
  • shortness of breath
  • swelling of feet or lower legs
  • swollen lymph nodes
  • swollen salivary glands
  • symptoms of liver damage (e.g., yellow skin or eyes, abdominal pain, dark urine, clay-coloured stools, loss of appetite, nausea and vomiting, or itching)

Stop taking the medication and seek immediate medical attention if any of the following occur:

  • cough
  • fever or chills
  • general feeling of discomfort, illness, or weakness
  • headache
  • hoarseness
  • mouth sores
  • pain, swelling, or redness in joints
  • skin rash
  • sore throat

Some people may experience side effects other than those listed. Check with your doctor if you notice any symptom that worries you while you are taking this medication.

Are there any other precautions or warnings for this medication?

Before you begin using a medication, be sure to inform your doctor of any medical conditions or allergies you may have, any medications you are taking, whether you are pregnant or breast-feeding, and any other significant facts about your health. These factors may affect how you should use this medication.

Blood tests: Your doctor will likely want you to have laboratory tests done on a regular basis because in rare cases, methimazole can cause the levels of white blood cells to decrease. Low white blood cell levels can increase the risk of infection. If you have a sore throat, skin rash, fever, headache, or a general feeling of being unwell, tell your doctor immediately.

Pregnancy: This medication should not be used during pregnancy unless the benefits outweigh the risks. If you become pregnant while taking this medication, contact your doctor immediately.

Breast-feeding: Women using methimazole should not breast-feed.

What other drugs could interact with this medication?

There may be an interaction between methimazole and any of the following:

  • amiodarone
  • beta-blockers (e.g., atenolol, metoprolol, propranolol)
  • digoxin
  • potassium iodide
  • sodium iodide
  • theophyllines (e.g., aminophylline, oxtriphylline, theophylline)
  • warfarin

If you are taking any of these medications, speak with your doctor or pharmacist. Depending on your specific circumstances, your doctor may want you to:

  • stop taking one of the medications,
  • change one of the medications to another,
  • change how you are taking one or both of the medications, or
  • leave everything as is.

An interaction between two medications does not always mean that you must stop taking one of them. Speak to your doctor about how any drug interactions are being managed or should be managed.

Medications other than those listed above may interact with this medication. Tell your doctor or prescriber about all prescription, over-the-counter (non-prescription), and herbal medications you are taking. Also tell them about any supplements you take. Since caffeine, alcohol, the nicotine from cigarettes, or street drugs can affect the action of many medications, you should let your prescriber know if you use them.